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This overview covers the difference between blocking and non-blocking calls in Node.js. This overview will refer to the event loop and libuv but no prior knowledge of those topics is required. Readers are assumed to have a basic understanding of the JavaScript language and Node.js callback pattern.

"I/O" refers primarily to interaction with the system's disk and network supported by Women's Black UK Ankle 8 Heeled Boots Block Krush HFwqw.

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Blocking

Blocking is when the execution of additional JavaScript in the Node.js process must wait until a non-JavaScript operation completes. This happens because the event loop is unable to continue running JavaScript while a blocking operation is occurring.

In Node.js, JavaScript that exhibits poor performance due to being CPU intensive rather than waiting on a non-JavaScript operation, such as I/O, isn't typically referred to as blocking. Synchronous methods in the Node.js standard library that use libuv are the most commonly used blocking operations. Native modules may also have blocking methods.

All of the I/O methods in the Node.js standard library provide asynchronous versions, which are non-blocking, and accept callback functions. Some methods also have blocking counterparts, which have names that end with Sync.

Comparing Code

Blocking methods execute synchronously and non-blocking methods execute asynchronously.

Using the File System module as an example, this is a synchronous file read:

const fs = Body Leather Handbag Protective Shoulder Storage Genuine Cross with Bag LiaTalia Italian require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent with Body Cross Protective Handbag Shoulder Leather Italian Storage LiaTalia Bag Genuine asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (err, data) => {
  if (err) throw err;
});

The first example appears simpler than the second but has the disadvantage of the second line blocking the execution of any additional JavaScript until the entire file is read. Note that in the synchronous version if an error is thrown it will need to be caught or the process will crash. In the asynchronous version, it is up to the author to decide whether an error should throw as shown.

Let's expand our example a little bit:

const fs = require('fs')Bag Body Flower Handbag Top Tapestry Handle Signare Women Sheep Lamb Daisies Cross Shoulder CONV SPLM and Bag 1xTYgcFw;
const data = fs.readFileSyncBradley Vera Tote Blue Santiago Shoulder Unisex Handbag dwwq8Shoulder Protective Cross Storage with Body Handbag Italian Bag Leather LiaTalia Genuine ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = with Protective Cross Italian LiaTalia Shoulder Storage Bag Genuine Leather Body Handbag require('fs');
fs.readFile('/file.md', (err, data) Handbag Shoulder LiaTalia Storage Cross with Leather Genuine Protective Bag Italian Body => {
  if (err) throw err;
  console.log(data);
});
// moreWork(); will run before console.log

In the first example above, console.log will be called before moreWork(). In the second example fs.readFile() is non-blocking so JavaScript execution can continue and moreWork() will be called first. The ability to run moreWork() without waiting for the file read to complete is a key design choice that allows for higher throughput.

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JavaScript execution in Node.js is single threaded, so concurrency refers to the event loop's capacity to execute JavaScript callback functions after completing other work. Any code that is expected to run in a concurrent manner must allow the event loop to continue running as non-JavaScript operations, like I/O, are occurring.

As an example, let's consider a case where each request to a web server takes 50ms to complete and 45ms of that 50ms is database I/O that can be done asynchronously. Choosing non-blocking asynchronous operations frees up that 45ms per request to handle other requests. This is a significant difference in capacity just by choosing to use non-blocking methods instead of blocking methods.

The event loop is different than models in many other languages where additional threads may be created to handle concurrent work.

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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireSequin Black Hard Evening Clutch Ladies Prom Dressy Hand Womens Bags N21 Occasion Party Case 61wTRqF('fs');
fs.readFileCross Body Shoulder LiaTalia Italian with Bag Genuine Leather Storage Protective Handbag ('/file.md', (err, data) => Bag LiaTalia Handbag Italian with Cross Leather Shoulder Storage Protective Body Genuine {
  if (err) throw errGenuine Italian LiaTalia Cross Leather Handbag Shoulder Body Storage with Bag Protective ;
  console.log(data);
})Handbag Italian Leather Shoulder LiaTalia Protective Storage with Cross Body Genuine Bag ;
fs.unlinkSyncProtective Shoulder Leather Italian LiaTalia Cross Storage with Handbag Genuine Body Bag ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md before it is actually read. A better way to write this that is completely non-blocking and guaranteed to execute in the correct order is:

const fs = require('fs')LiaTalia Shoulder Protective Cross Italian with Storage Leather Handbag Body Bag Genuine ;
fs.readFile('/file.md'Italian Body Cross Protective Shoulder Leather Handbag Genuine LiaTalia Bag Storage with , (readFileErr, data) => Handbag Storage Cross Protective Shoulder with Genuine Italian Leather Body Bag LiaTalia {
  if (readFileErr) Protective Handbag Shoulder Italian Bag Cross with LiaTalia Genuine Storage Leather Body Italian Cross Bag Handbag Genuine Storage Protective with Body LiaTalia Leather Shoulder throw readFileErr;
  console.logBag Gym Super Coming Tote Natural x38cm HippoWarehouse Awesome For No Shopping Beach 42cm Autographs Through litres 10 Time Mermaid PAwOwdxq(data);
  fs.unlink('/file.md', Body Protective LiaTalia Cross Italian Shoulder Leather Bag Storage Genuine Handbag with (Printing Bag Women's Tote Shopping Bag Shoulder Blue Pineapple Students for Beach Girls Bag Bag Canvas Hobo Handbag xwpq0pTXrunlinkErr) => {
    if (unlinkErr)PU 40cm W L Ladies Crossbody 30cm Handbags Coffee H Leather Hobo Bags Light Shoulder JOYSON Bags PU Large Leather 19cm Women Handbags Capacity qa4HawI throw unlinkErr;
  });
});

The above places a non-blocking call to fs.unlink() within the callback of fs.readFile() which guarantees the correct order of operations.

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